In recent years, the amount of data to be transmitted between the ground, and an aircraft or an artificial satellite is increasing. Further, in order to implement large-capacity data communication, a free space optics (FSO) system using an optical frequency band capable of significantly increasing a band as compared with a microwave is being investigated.
Among such free space optics (FSO) systems, in an FSO system in which communication is performed between the ground and a low earth orbit (LEO) satellite, there are constraints on communication time. Consequently, it is important to stably maintain tracking with increasing a bit rate. The reason is that, if tracking cannot be maintained, the communication capacity of an FSO system is reduced because communication time is reduced by time required for recapturing.
In order to maintain stable tracking in a free space optics (FSO) system, it is necessary to transmit a beacon beam stably. In order to implement stable transmission of a beacon beam, it is necessary to solve the following two technical challenges. The first challenge is to make it possible to receive a high-sensitive beacon beam suppressing background light. The second challenge is to mitigate the impact of atmospheric turbulence on the beacon beam. Each of the challenges will be described below.
The first challenge is that it is necessary to remove reflected light from the sun, the moon, or the earth, which is called background light, from light collected by a receiving telescope. This is because, if background light is received simultaneously with a beacon beam, a receiving signal/noise (S/N) ratio of the beacon beam is lowered. Specifically, it becomes difficult to detect a high-sensitive beacon beam due to the saturation of an optical receiver or the increase in beat noise caused by background light; consequently, tracking becomes unstable.
Whereas a laser device with a narrow linewidth is used for the beacon beam as a light source, the background light has broadband continuous spectral components having their origin in sunlight. Accordingly, it is possible to improve the receiving S/N ratio of the beacon beam by blocking the spectral components of the background light using a narrowband optical band-pass filter (BPF) that lets the light with only a band of the beacon beam through.
Ideally, it is desirable to keep the passband width of the optical band-pass filter (BPF) used here as narrow as possible. However, in the free space optics (FSO) system between the ground and an artificial satellite, it is necessary to consider a frequency shift of laser light due to Doppler effect. Specifically, a normalized amount of a Doppler shift between the ground and a low earth orbit satellite is about ±3×10−5, for example. This results in an occurring shift amount of about ±6 GHz when using laser light having the wavelength of 1.55 micrometer (μm), that is, the frequency of about 200 terahertz (THz).
However, it is not preferable to mount, in an artificial satellite, an optical band-pass filter (BPF) that variably controls a passing center frequency following a Doppler shift because it leads to an increase in power consumption and device weight. It is therefore necessary, in order to deal with such a Doppler shift, to use an optical band-pass filter (BPF) having a passing bandwidth of about 18 GHz (wavelength width is about 0.14 nm) that is about 1.5 times as wide as a shift amount, for example, in consideration of a margin. Such an optical band-pass filter (BPF) can be obtained by combining a spatial Bragg grating filter and an etalon, for example.
Using such a narrowband optical band-pass filter (BPF) makes it possible to remove background light sufficiently and receive a beacon beam with high sensitivity. On the other hand, applying a narrowband optical band-pass filter (BPF) is a limiting condition for the spectrum of laser light used for a beacon beam.
Next, the second challenge will be described. The second challenge is that it is necessary to stabilize a variation in the received light intensity of a beacon beam that arises on a receiving side from wavefront disturbance of the beacon beam due to propagation through the atmosphere. If the intensity of a beacon beam to be received largely fades due to strong atmospheric turbulence, it becomes difficult to control a tracking precisely because an S/N ratio of an error signal detected by a tracking control system is degraded. This particularly becomes conspicuous if a beacon beam is transmitted from the ground to an artificial satellite overhead. This is because the beacon beam propagating from the ground toward an artificial satellite is strongly affected by atmospheric turbulence. That is to say, this is because the beacon beam transmitted from the ground is affected by atmospheric turbulence immediately after the transmission, propagates over a long distance in a vacuum without atmospheric turbulence maintaining a spatial intensity distribution, and is enlarged and projected on an orbital plane of the artificial satellite.
If the intensity distribution of the beacon beam is enlarged in a plane, it is impossible to obtain the aperture averaging effect in a receiving-side telescope, and the beacon beam is strongly affected by atmospheric turbulence. If strong fade occurs due to atmospheric turbulence, and a beacon disappears, the tracking control system of an artificial satellite loses a position of a ground station. As a result, it becomes impossible to radiate a signal beam accurately from the artificial satellite toward the ground, and it becomes difficult to perform stable free space optics (FSO).
A size of a spatial intensity distribution of a beacon beam will be described below using a specific example.
If it is assumed that a beacon beam propagating from the ground toward a satellite is a spherical wave, a coherence radius of the intensity distribution is expressed by following formula (1).
                              ρ                      0            ,            sph                          =                              [                          1.46              ⁢                              k                2                            ⁢                                                ∫                  0                  L                                ⁢                                                                                                    C                        n                        2                                            ⁡                                              (                        z                        )                                                              ·                                                                  (                                                  z                          ⁢                                                      /                                                    ⁢                          L                                                )                                                                    5                        ⁢                                                  /                                                ⁢                        3                                                                              ⁢                  dz                                                      ]                                              -              3                        ⁢                          /                        ⁢            5                                              (        1        )            
Because formula (1) includes a propagation distance L in its numerator, the spatial size of the beacon beam intensity distribution increases as the propagation distance increases. As a result of estimating a coherence radius ρ0, sph from formula (1) using a type of Hufnagle-Valley (HV) model as typical structural parameters of the atmosphere, the result of ρ0, sph=6.4 m is obtained if a typical propagation distance for a low earth orbit satellite L is set at 600 km. Given that a spatial size of the intensity distribution is equivalent to a spatial coherent radius, it is impossible to obtain the aperture averaging effect unless the diameter of a telescope on a satellite side is equal to or larger than 13 m. However, there has been the problem that mounting such a giant telescope on a satellite side increases the cost due to an increase in weight and volume.
Patent Literature 1 and Patent Literature 2 disclose examples of technologies to solve such problems caused by the atmospheric turbulence.
A multi-beam laser communication device described in Patent Literature 1 includes first to fourth telescopes for laser beam transmission, a laser pointing device, a telescope for receiving light, a gimbal mechanism to adjust an azimuth and an elevation angle for transmitting and receiving, and a controller. The controller selects a laser light source to be used depending on beam conditions to radiate a laser beam from a telescope, and adjusts a beam divergence angle of the laser beam so as to suppress a variation in received light intensity on the other side. This makes it easy to hold a laser line in an environment that there are atmospheric turbulence and a pointing error, which is described in Patent Literature 1.
Patent Literature 2 discloses a free space optical transmitter having a transmitting-side station that includes a plurality of signal light sources to radiate signal beams whose wavelengths differ from each other, a drive circuit to modulate each signal light source by an inputted electrical signal, a mirror to multiplex respective signal beams on a same optical axis, and a beam splitter. This configuration makes it possible to transmit a same signal simultaneously by a plurality of signal beams whose wavelengths differ from each other; consequently, it is possible to reduce a variation in received optical power on a receiving side as compared with a transmission using one light source, which is described in Patent Literature 2.
There is another technology described in Patent Literature 3 as related technologies.